1. Field of the Invention
The present invention relates to a brake control system for an electrically operated vehicle to perform a regenerative braking operation and a hydraulic braking operation, and more particularly to the brake control system which is adapted to reduce a regenerative braking torque and add a hydraulic braking force to compensate for lack of the regenerative braking torque, when voltage of a battery exceeds a predetermined upper limit, thereby to protect the battery.
2. Description of the Related Arts
In an electrically operated vehicle provided with an electric motor as a power source, the electric motor also functions as a generator for performing the regenerative braking operation to recover energy by charging a battery and use the energy for driving the motor. Since the braking force applied to the vehicle wheels through the regenerative braking operation is limited, it must be compensated by a hydraulic braking operation. Therefore, both of the regenerative braking operation and the hydraulic braking operation are performed, as disclosed in Japanese Patent Laid-open Publication No.5-161210. According to the prior brake control system for the electrically operated vehicle as disclosed in the Publication No.5-161210, when the voltage of the battery increases to exceed a predetermined threshold level, the regenerative braking torque is reduced so as to protect the battery, whereby the braking force through the regenerative braking operation is reduced. However, when the control for reducing the regenerative braking torque is initiated, if the regenerative braking torque is reduced rapidly, the compensation through the hydraulic braking operation will be delayed, so that deceleration of the vehicle may be reduced temporarily. As a result, feelings to a vehicle driver in his braking operation will be varied.
Also, according to the prior brake control system as disclosed in the Japanese Patent Laid-open Publication No. 5-161210, the regenerative braking torque is reduced temporarily, so that if the voltage of the battery decreases to enable the regenerative braking operation, it will be restarted. In this case, there remains the braking force such as the hydraulic braking force which compensated for the lack of the braking force when the regenerative braking torque was reduced. Therefore, the deceleration of the vehicle will be increased by the restarted regenerative braking operation. In other words, additional braking force will be applied against the vehicle driver's intention. As a result, feelings to the vehicle driver in his braking operation will be deteriorated.